Apparatus with permanent magnetic lenses

ABSTRACT

The invention describes a particle-optical apparatus arranged to focus a beam ( 1 ) of electrically charged particles with the aid of two particle-optical lens systems ( 10, 20 ). The lens action is achieved by magnetic fields, which fields are generated by permanent-magnetic materials ( 13, 23 ). In contrast to magnetic lenses equipped with a coil, it is not easy in the case of lenses equipped with permanent-magnetic material to alter the focusing magnetic field with the aim of altering the optical power. In an apparatus according to the invention, the optical power of the lens systems is altered by altering the energy with which the beam ( 1 ) traverses the lens systems ( 10, 20 ). This can easily happen by altering the voltage of electrical power supplies ( 14, 24 ).

The invention relates to a particle-optical apparatus arranged to:

-   -   Focus a beam of electrically charged particles with the aid of a        first particle-optical lens system and a second particle-optical        lens system;    -   In which lens system the lens action is at least partially        realized by magnetic fields;    -   Which magnetic fields are generated by permanent-magnetic        materials.

Such an apparatus is known from U.S. Pat. No. 6,320,194.

Apparatus as described above are used nowadays inter alia in studyingsamples. In irradiating a sample with a focused beam of chargedparticles, information can be obtained in various manners, such as withthe aid of secondary particles and radiation locally excited in thesample. By collecting and processing this information with the aid ofdetectors, insight is obtained into certain physical properties of thesample. Because the beam can have a very small diameter, the positionaldependence of this information can be determined with a high spatialaccuracy.

To collect this information, it is desirable to be able to vary beamparameters such as the beam current, beam energy and beam diameter atthe location of the sample. This provides the possibility of, forexample, first localizing on the sample a microscopically smallstructure that is to be investigated, e.g. with the aid of a relativelyhigh beam current, and subsequently studying this structure with anotherbeam energy or current.

As is known to the skilled artisan, the beam in a particle-opticalapparatus is focused by particle-optical lens systems. The magneticand/or electric fields present in such a lens system have a focusingaction on the beam. Particularly in those cases in which the beamconsists of electrons, a magnetic lens system is usually employed,because, in using such a lens system, the lens errors are generallysmaller than the lens errors that occur in the case of an electrostaticlens system. By now varying the optical power of such a lens system, thepossibility arises of varying the beam parameters.

The optical power of such a magnetic lens system is varied by varyingthe strength of the magnetic field. To this end, the magnetic field isusually generated by an electromagnetic coil. The current necessary togenerate the magnetic field will cause heat dissipation in the coil. Thephysical dimensioning of the magnetic lens system is determined in largepart by the size of the coil and the space required for any coolingmeans that may be employed. These cooling means, such as a water coolingspiral, may be necessary to limit undesired consequences of thedissipation, such as mechanical changes as a result of the temperaturechanges of the pole pieces that guide the magnetic field.

The use of permanent-magnetic material to generate the magnetic field ina magnetic lens system has the advantage that a more compact magneticlens system can be made, inter alia because no heat dissipation occursin this case. However, when using permanent-magnetic material, it is notpossible to vary the magnetic field in a simple manner.

In the field of particle-optical apparatus, it is desirable to have acompact particle-optical apparatus. There is also a need for flexibleparticle-optical systems, in which beam parameters can be varied in asimple and fast manner.

In said US patent document, a particle-optical apparatus is describedthat is equipped with a particle-optical column. This column comprisesan electron source and a condenser lens system equipped withpermanent-magnetic material. Although it cannot be directly derived fromthe patent document, it is customary for the skilled artisan toelectrically connect the various parts of the magnetic circuit and thesample to earth potential. The beam of electrons emerging from theelectron source is accelerated to a desired energy. Thereafter, the beamis focused on the sample using magnetic lens systems, i.e. the condenserlens system and an objective lens system. Using scanning coils (notdepicted), the focused beam is moved across a sample, whereby emergingradiation is detected with the aid of a detector.

The desired flexibility is achieved in the known apparatus by making atleast the electron source and the condenser lens system mechanicallyexchangeable, i.e. by replacing the electron source and the condenserlens system by another electron source and another condenser lenssystem. If it is desired to conduct inspection using another beamparameter, a portion of the column is replaced by a portion of thecolumn with other optical properties. Moreover, it is shown how a finecontrol of the magnetic field is made possible via mechanicaladjustments to the magnetic circuit (the co-called “bypass circuit”) andby addition of relatively small coils.

The exchange of the electron source and the condenser lens system asreferred to in said US patent document does indeed offer the desiredflexibility of the beam parameters, but is less suitable when it isdesired to irradiate a sample using different beam parameters in a shortspan of time. Said exchange is a relatively laborious solution, wherebyvariation of the beam parameters requires considerable time compared tothe time necessary to study the sample. Moreover, in the knownapparatus, there is a danger that, after the (mechanical) exchange ofthe electron source and the condenser lens system, the position of thebeam with respect to the sample is insufficiently known to allow easyre-location of a microscopically small structure on the sample that wasfound prior to said exchange.

The invention aims to provide a particle-optical apparatus with acompact column, whereby beam parameters can be varied in a simple andfast manner.

To this end, an apparatus according to the invention is characterized inthat the beam traverses the first lens system with an energy differentto the energy with which the beam traverses the second lens system.

The invention is based upon the inventive insight that it is possible toachieve the desired flexibility of the lens systems, not by varying thefocusing magnetic field of the lens systems, but by varying the energyof the beam as it traverses these focusing magnetic fields. This isbecause the optical power of a magnetic lens system is not onlydependent upon the focusing magnetic field, but is also dependent uponthe energy of the beam at the location of the focusing magnetic field.By suitably choosing the energy of the beam as it traverses the magneticlens system, it is possible to adjust the optical power.

The energy with which the beam traverses a lens system can be varied byaccelerating or retarding the beam with the aid of an electrostaticfield prior to traversal of the lens system. In traversing theelectrostatic fields that cause a retardation or acceleration, a lensaction will also occur, as is known to the skilled artisan. The lensactions of the magnetic and electrostatic fields together determine thebeam parameters.

In another embodiment of the apparatus according to the invention, thebeam, in traversing the first lens system, moves within a first tube ofelectrically conductive material that is surrounded by the first lenssystem, and, in traversing the second lens system, moves within a secondtube of electrically conductive material that is surrounded by thesecond lens system.

The two tubes of electrically conductive material determine the energyof the beam within those tubes, and thereby the energy with which thebeam traverses the focusing magnetic fields. The tubes are electricallyisolated from each other and from earth, so that they can carrydifferent voltages.

It should be mentioned that the two tubes have to be made of anon-magnetizable material, so that the tubes will not hinder penetrationof the focusing magnetic field as far as the axis.

As is known to the skilled artisan, it is necessary for the beam topropagate through a vacuum. Said tube can function as a vacuum barrier,whereby vacuum prevails within the tube and a higher pressure, e.g.atmospheric pressure, prevails outside the tube.

An advantage of this embodiment is that the permanent-magnetic material,which generates the magnetic field, and any pole pieces that arepresent, which serve to guide the magnetic flux produced by thepermanent-magnetic material toward the beam, are located outside ofvacuum. In this way, no special demands are made as regards the vacuumcompatibility of the material of the pole pieces and the permanentmagnetic material.

In yet another embodiment of the apparatus according to the invention,each of the lens systems is provided with a flux guiding circuit forguiding the flux generated by the permanent magnetic materials, each ofwhich flux guiding circuits carries a different electrical voltage.

In this embodiment, the flux guiding circuits are located at leastpartially in the vacuum in which the beam propagates. The flux guidingcircuits comprise pole pieces of magnetizable material. By connectingeach of these pole pieces to a corresponding electrical voltage source,these pole pieces determine the energy with which the beam traverses thefocusing magnetic fields of the lens system concerned.

An advantage of this embodiment is that, by using the pole pieces bothto guide the magnetic field and to determine the energy of the beam, itis possible to make a very compact lens system. This makes it possibleto realize a compact particle-optical apparatus.

In yet another embodiment of the apparatus according to the invention, avoltage difference prevails between parts of the flux guiding circuit ofat least one of the lens systems.

The energy of the beam varies during traversal of the lens systemconcerned. As a result of this, a change in optical power occurs w.r.t.the situation whereby no voltage difference is applied between the partsof the flux guiding circuit.

An advantage of this embodiment is that it is, for example, possible toassign to one of the pole pieces a fixed voltage w.r.t. earth, and yetis also possible to vary the optical power of the lens system. This isparticularly attractive in the case of the pole piece that is closest tothe sample to be investigated. This is because, in connection with thedetection of, for example, secondary electrons, it is often desirablethat the sample and the pole piece closest thereto be connected toearth. It is also possible, for example, where use is made of anelectron source in the form of an electron field emitter, to employ thepole piece that is closest to the field emitter as an extractionelectrode.

In yet another embodiment of the apparatus according to the invention,the magnetization direction of the permanent-magnetic material in thetwo lens systems is mutually chosen in such a manner that, between thetwo lens systems, there is located a plane in which there is essentiallyno magnetic flux present parallel to an axis passing through the middleof the lens systems.

A complication in designing particle-optical lens systems withpermanent-magnetic material is that, as a result of usingpermanent-magnetic material, it is possible for scatter fields to occuroutside the lens system. As is known to the skilled artisan, this is adirect consequence of Ampere's law, which states that the closed-loopintegral o∫Bds is zero in a situation whereby no current-carryingportions are enclosed, where B is the magnetic field and s is a (closed)path. So as to cancel the effects of said scatter fields, which areusually undesirable, it is customary to magnetically shield the beam bysurrounding the beam by magnetizable material to as great an extent aspossible. However, such surrounding by magnetizable material isgenerally undesirable, since this leads to a more complex construction.This is also undesirable because, as a consequence, access to regions inthe vicinity of the beam, e.g. so as to be able to place and operatevacuum valves there, is made more difficult.

By suitably choosing the direction of magnetization in thepermanent-magnetic material in one lens system w.r.t. the magnetizationdirection in the other lens system, it is possible to compensate thescatter field of the first lens system with the scatter field of thesecond lens system. In this way, it is possible to identify between thetwo lens systems a plane where there is no magnetic flux presentparallel to the axis passing through the middle of the two lens systems.In the space around this plane, it is possible to remove themagnetizable material without, as a consequence, significant change tothe magnetic fields on the axis. This is because, if there is nomagnetic flux, the presence of magnetizable material will not berelevant.

The absence of magnetizable material in a space between the lens systemsnow makes it simple to mutually electrically isolate the lens systems,so that a voltage difference can be applied between the lens systems.

Another advantage occurs when the particle-optical apparatus is of atype whereby the sample is positioned between the lenses, as in the caseof a Transmission Electron Microscope (TEM). It is then attractive toplace the sample in a space that is essentially free of magnetic fields,so that magnetization of the sample and sample holder (probe) will nothave any influence on image formation in the particle-optical apparatus.However, it is also desirable to have good spatial access to the sampleposition, so as to be able to place, for example, detectors and/orcooling means there. As a result of the presence of a region in whichthere is essentially no flux parallel to the axis passing through themiddle of the two lens systems, it is easy to satisfy both of thesedesires.

The invention will be elucidated on the basis of figures, wherebyidentical reference numerals indicate corresponding elements. To thisend:

FIG. 1 schematically depicts a particle-optical apparatus according tothe invention, whereby the beam runs inside a tube;

FIG. 2 schematically shows how the flux runs in both lens systems; and

FIG. 3 schematically illustrates a particle-optical apparatus accordingto the invention, whereby the pole pieces are at an electrical voltage.

FIG. 1 schematically depicts a particle-optical apparatus according tothe invention, whereby the beam runs inside a tube.

An electron source 2 and a sample 3 to be investigated are located on anoptical axis 5. Between the electron source 2 and the sample 3 arelocated a first lens system 10 and a second lens system 20, which lenssystems 10 and 20 demonstrate rotational symmetry about the optical axis5.

The electron source is connected to an electrical power supply 4, withwhich the electron source is maintained at an electrical voltage w.r.t.earth. Lens system 10 is connected to an electrical power supply 14 andis kept at a voltage w.r.t. the electron source 2. In the same way,electrical power supply 24 is used to keep lens system 20 at a voltagew.r.t. the electron source.

The first lens system 10 comprises a tube 15 disposed around the opticalaxis 5. Around this tube 15, a ring 13 of permanent-magnetic material isplaced, whose magnetization is oriented parallel to the axis 5. Polepieces 11 and 12 are mounted on this ring 13, so as to guide themagnetic flux generated by the ring 13 toward a region 17 around theoptical axis 5. In this region 17, the flux will cross over from onepole piece to the other, and thereby cause a magnetic field about theaxis 5. Because the pole pieces 11 and 12 are not allowed to bemagnetically connected to one another (this would, after all, cause amagnetic short-circuit of the ring 13 of permanent-magnetic material),flux also crosses over from one pole piece to the other in a region 16,in addition to region 17.

In the same way, the second lens system 20 comprises a tube 25 disposedaround the optical axis, a ring 23 of permanent-magnetic material, andpole pieces 21 and 22, whereby the flux crosses over from one pole pieceto the other in the regions 26 and 27.

The tubes 15 and 25 are electrically isolated from one another by aninsulator 6, so that a voltage difference may exist between the tubes.

A beam 1 of electrically charged particles, such as electrons, isemitted along the optical axis 5 from a particle source such as theelectron source 2. The beam 1 is accelerated by the voltage differencebetween the electron source 2 and the tube 15, as caused by theelectrical power source 14. The beam 1 is focused by the magnetic fieldpresent in region 17, pertaining to lens system 10. As a result of thevoltage difference between the tubes 15 and 25, the energy of the beam 1will alter in going from tube 15 to tube 25. With this altered energy,which is determined by the electrical power supply 24, the beam 1 willbe focused by the magnetic field present in region 27, pertaining tolens system 20. Eventually, the beam 1 will intercept the sample 3 witha landing energy that is determined by the electrical power supply 4.Radiation, such as secondary electrons, will hereby be generated. Thisradiation can be detected using (non-depicted) detectors.

Alteration of beam parameters can be easily achieved in this embodiment.When, for example, in investigating the sample 3, it is necessary toraise the landing energy with which the beam 1 intercepts the sample 3,this can be easily achieved by raising the voltage of the electricalpower supply 4. The energy with which the beam 1 traverses the lenssystems 10 and 20 will not change as a result of changing the voltage ofpower supply 4, and, accordingly, the focusing action of the magneticfields of the lens systems 10 and 20 will also not change.

As a result of this alteration of the landing energy, the lens action ofthe electrostatic fields present can change. This is because themagnetic and electrostatic fields together determine the beamparameters. This change in the lens action of the electrostatic fieldscan be compensated for by a small alteration to the focusing action ofthe magnetic field of, for example, lens system 20, by slightly varyingthe voltage of the electrical power supply 24.

Likewise, the beam parameters can be altered—while keeping constant, forexample, the energy with which the beam 1 intercepts the sample 3—byletting lens system 10 focus more strongly, by lowering the energy ofthe beam, and, concurrently, by letting lens system 20 focus lessstrongly, by raising the energy of the beam in lens system 20.

Focusing of the beam 1 upon the sample 3 can also occur by varying thevoltage of the electrical power supply 24, and thereby the energy withwhich the beam traverses lens system 20. The landing energy will notchange as a result of such action.

It should be mentioned that it is also possible to connect the voltagesources 14 and 24 on one side to, for example, earth. Only the voltagethat is present on the tubes 15 and 25 is of importance.

FIG. 2 schematically shows how the flux runs in both lens systems.

In region 17, the flux crosses over from pole piece 11 to pole piece 12.The axially symmetric magnetic field thereby generated causes thedesired lens action of the lens system 10. Where the lens action isconcerned, the polarity of the magnetic field is unimportant. Besides inregion 17, flux will also cross over in region 16.

Likewise, the magnetic field that causes the lens action of the lenssystem 20 is generated in region 27. In this case also, the polarity ofthe magnetic field is unimportant. Besides in region 27, flux will alsocross over in region 26.

By choosing the mutual magnetization of the lens systems 10 and 20 insuch a manner that the radial component of the flux in region 16 has thesame direction as the radial component of the flux in region 26, one canprevent the cross-over of flux between the lens systems 10 and 20.Because no flux crosses over between the lens systems 10 and 20, it isalso not necessary to have magnetizable material between the lenssystems so as to guide the flux. As a result, good spatial accessibilityof the region between the lens systems 10 and 20 is made possible. Thisis advantageous when, in this space, (non-depicted) vacuum valves haveto be placed and operated.

It should be mentioned that, although in the depicted figure, as aresult of the employed mirror symmetry w.r.t. the symmetry plane 7, itcan easily be seen that no flux crosses over between the lens systems 10and 20, a plane between the lens systems 10 and 20 where there is noflux cross-over will also be present in embodiments in which thissymmetry is absent.

FIG. 3 can be regarded as having arisen from FIG. 1, whereby the energyof the beam 1 is not determined by the electrical voltages of the tubes15 and 25 depicted in FIG. 1, but is determined by the electricalvoltages that are present on the pole pieces 11, 12, 21 and 22.

The electrical power supply 14 causes a voltage difference betweenelectron source 2 and the pole pieces 11 and 12. As a result of thisvoltage difference, the energy of the beam is determined in traversingthe focusing magnetic field of lens system 10.

Lens system 20 additionally has an electrical insulator 28. As a resultof this, it is possible to give pole piece 21 a different electricalvoltage than pole piece 22. By connecting pole piece 21 to theelectrical power supply 24 and connecting pole piece 22 to earth, asituation is achieved whereby the (average) energy with which the beamtraverses the focusing magnetic field of lens system 20 is determined bythis power source and power source 4. Because both pole piece 22 and thesample 3 are connected to earth, the space between pole piece 22 and thesample 3 will be a space free of electrical fields. This lattersituation is advantageous because electrical fields in this space mighthinder, or at least complicate, the detection of radiation andelectrically charged secondary particles by (non-depicted) detectors.

It should be mentioned that, although as depicted in FIGS. 1 and 3 thebeam 1 demonstrates a focus between the lens systems 10 and 20, this isgenerally not necessary.

1. A particle-optical apparatus arranged to: focus a beam ofelectrically charged particles with the aid of a first particle-opticallens system and a second particle-optical lens system; in which lenssystem the lens action is at least partially realized by magneticfields; which magnetic fields are generated by permanent-magneticmaterials, characterized in that the beam traverses the first lenssystem with an energy different from the energy with which the beamtraverses the second lens system.
 2. A particle-optical apparatusaccording to claim 1, in which the beam, in traversing the first lenssystem, moves within a first tube of electrically conductive materialthat is surrounded by the first lens system, and, in traversing thesecond lens system, moves within a second tube of electricallyconductive material that is surrounded by the second lens system, whichtubes carry mutually different electrical voltages.
 3. Aparticle-optical apparatus according to claim 1, in which each of thelens systems is provided with a flux guiding circuit for guiding theflux generated by the permanent magnetic materials, each of which fluxguiding circuits carries a different electrical voltage.
 4. Aparticle-optical apparatus according to claim 3, in which an electricalvoltage difference prevails between parts of the flux guiding circuit ofat least one of the lens systems.
 5. A particle-optical apparatusaccording to claim 1, in which the magnetization directions of thepermanent-magnetic materials in the two lens systems are mutually chosenin such a manner that, between the two lens systems, there is located aplane in which there is essentially no magnetic flux present parallel toan axis passing through the middle of the lens systems.
 6. Aparticle-optical apparatus according to claim 2, in which themagnetization directions of the permanent-magnetic materials in the twolens systems are mutually chosen in such a manner that, between the twolens systems, there is located a plane in which there is essentially nomagnetic flux present parallel to an axis passing through the middle ofthe lens systems.
 7. A particle-optical apparatus, comprising; a sourceof charged particles; a first particle-optical lens system including afirst permanent magnet; a second particle-optical lens system includinga second permanent magnet; and at least one element within the firstparticle-optical lens system or the second particle-optical lens systemto which a voltage can be applied to alter the electric field within thecorresponding one of the first or second lens system; a sample holderfor holding a sample, wherein the electrical potential within the firstparticle-optical lens system, within the second particle-optical lenssystem, or within both particle-optical lens systems being adjustableindependently of the electrical potential between the source of chargedparticles and the sample.
 8. A charged particle apparatus according toclaim 7 in which the electrical potential within the firstparticle-optical lens system and the electrical potential within thesecond particle-optical lens system can be adjusted independently ofeach other.
 9. A charged particle apparatus according to claim 7 inwhich the at least one element to which a voltage can be appliedincludes an electrically conductive tube positioned so that the chargedparticles travel through the tube.
 10. A charged particle apparatusaccording to claim 7 in which the at least one element to which avoltage can be applied includes a magnetic flux guide for guiding theflux from the first permanent magnet or from the second permanentmagnet.
 11. A charged particle apparatus according to claim 7 in whichthe at least one element to which a voltage can be applied includes atleast two elements, one element in each of the first and second lenssystems.
 12. A charged particle apparatus according to claim 11 in whichthe at least one element to which a voltage can be applied in each ofthe first and second lens systems includes a first electricallyconductive tube in the first lens system and a second electricallyconductive tube in the second lens system, the electrical potential ofthe first and second tubes being independently adjustable.
 13. A chargedparticle apparatus according to claim 11 in which the at least oneelement to which a voltage can be applied in each of the first andsecond lens systems includes at least one first flux guide for guidingthe flux generated by the first permanent magnet and at least one secondflux guide for guiding the flux generated by the second permanentmagnet.
 14. A charged particle apparatus according to claim 13 in whichthe at least one first flux guide includes a first set of two first fluxguides and in which the at least one second flux guide includes a secondset of two second flux guides, each of the two first flux guides beingat substantially the same first voltage and each of the two second fluxguides being at substantially the same second voltage, the secondvoltage being different from the first voltage.
 15. A charged particleapparatus according to claim 7 in which the magnetization directions ofthe first permanent magnet and the second permanent magnet are mutuallychosen in such a manner that, between the two lens systems, there islocated a plane in which there is essentially no magnetic flux presentparallel to an axis passing through the middle of the lens systems. 16.A charged particle apparatus according to claim 7 in which the source ofcharged particles comprises a source of electrons.
 17. A method ofdirecting a charged particle beam toward a sample, comprising:generating a beam of charged particles from a charged particle source;directing the beam of charged particles through a first magnetic fieldof a first permanent magnet of a first lens systems and a secondmagnetic field of a second permanent magnetic of a second lens system;adjusting the beam properties by altering the voltage on an elementwithin the first lens system, the voltage on an element within thesecond lens system, or the voltage on elements within both lens systems.18. The method of claim 17 further comprising passing the chargedparticle beam through an electrically conductive tube within each of thefirst and second lens systems, and in which adjusting the beamproperties includes adjusting the voltage applied to one or both of theelectrically conductive tubes.
 19. The method of claim 17 in which eachof the first and second lens systems includes guides for the guidingmagnetic flux and in which adjusting the beam properties includesadjusting the voltage applied to one or both of the guides for themagnetic flux.
 20. The method of claim 17 in which directing the beam ofcharged particles through a first magnetic field of a first permanentmagnet of a first lens systems and a second magnetic field of a secondpermanent magnetic of a second lens system includes directing the beamof charged particles through a first and second lens system in which themagnetization directions of the permanent-magnetic materials in the twolens systems are mutually chosen in such a manner that, between the twolens systems, there is located a plane in which there is essentially nomagnetic flux present parallel to an axis passing through the middle ofthe lens systems.
 21. The method of claim 17 in which adjusting the beamproperties includes applying a first voltage to an element of the firstlens system and applying a different voltage to an element of the secondlens system.
 22. The method of claim 17 in which adjusting the beamproperties includes adjusting a voltage between the charged particlebeam source and the sample, without significantly altering the energy ofthe charged particles within the first and second lens system.
 23. Themethod of claim 17 in which adjusting the beam properties includesaltering the beam energy in the first or second lens system withoutsignificantly altering the landing energy of the charged particles. 24.A method of directing a charged particle beam toward a sample,comprising: generating a beam of charged particles from a chargedparticle source; directing the beam of charged particles through a firstmagnetic field of a first permanent magnet of a first lens systems and asecond magnetic field of a second permanent magnetic of a second lenssystem; adjusting the beam properties by altering the potential energywithin the first lens system, the second lens system, or within bothlens systems to vary the potential energy gradient from the uniformgradient produced by the potential applied between the charged particlebeam source and the sample.